This invention relates to the use of solutions of potassium formate to dehydrate gas by contacting one side of a membrane with the gas and the other side with the potassium formate solution. After absorbing moisture from the gas, the solution is regenerated and recirculated for reuse. The invention is particularly useful for dehydrating natural gas.
Glycols, especially triethylene glycol, are used to absorb moisture from natural gas, commonly by direct contact in a tower or the like. After absorbing water from the gas, the glycol solutions are usually regenerated by heating them to evaporate the water; the glycol is then returned to the water absorption unit to absorb more moisture. Inevitably, the environment is exposed to the glycol solutions used in gas dehydration. Glycols are generally environmentally undesirable. An article at pages 59-79 of Chapter 6, Section 2 of the Engineering Data Books of the Gas Producers Society of America, entitled xe2x80x9cGlycol Dehydrationxe2x80x9d, describes a typical gas drying and solution regeneration system, and some of the problems that can arise with such systems, such as pump failures, leaks, maintaining regeneration temperatures, flooding of the dehydrators, inefficient glycol compositions, plugged trays, and others. Nevertheless, such systems are widely used. A typical prior art gas drying and glycol regeneration system is shown in FIG. 1 hereof in a simplified form.
A more environmentally acceptable gas drying medium and process is needed. The water absorption medium should be efficient and readily regenerable with a minimum of maintenance, as many gas drying units are placed in remote locations.
Gas separation through membranes is generally known. In terms of structure, two general types of membrane separators are commonly usedxe2x80x94hollow fibers, which are usually unsupported, and self-supporting membranes or membrane films laid down on a permeable support; the support is usually either tubular or planar. Commonly the membrane is designed or selected to remove the components of interest efficiently while retaining other componentsxe2x80x94see for example Yamazaki U.S. Pat. No. 4,110,392. Porous membrane products are described by Gore in U.S. Pat. No. 4,187,390, Gore and Allen in U.S. Pat. No. 4,194,041, and Gore in U.S. Pat. No. 3,953,566. Removal of permeate from the permeate side is commonly assisted by a sweep gas.
The use of a membrane between a gaseous feed and a liquid absorbent is unusual. See, as examples, Jansen and Feron U.S. Pat. No. 5,749,941, Birbara and Nalette U.S. Pat. No. 5,281,254, and Falk-Pedersen and Dannstrom U.S. Pat. No. 6,228,145. The choice of a liquid absorbent for its ability to absorb the target component through a membrane is also rare. See Bowser and Dennison U.S. Pat. No. 5,382,364.
Potassium formate is proposed for use together with glycols in a countercurrent direct contact system by Gavlin and Goltsin in U.S. Pat. No. 5,725,637-see lines 25-32 of column 3. See also Gavlin and Goltsin U.S. Pat. No. 5,853,458. Atkinson, in U.S. Pat. No. 5,846,450, uses potassium formate solutions as absorbents in refrigeration systems.
This invention uses an aqueous alkali metal formate solution to dehydrate natural gas by placing the solution on one side of a membrane and the gas to be dehydrated on the other side. Moisture passes from the gas through the membrane and is readily absorbed by the alkali metal formate solution. In a preferred form of my invention, the membrane is supported on the outside, or shell side, of a permeable tubular surface and the alkali metal, preferably potassium, formate solution is circulated on the inside of the membrane-coated tube while the gas passes countercurrently in contact with the outside, or shell side, of the tube. The process can be run in the opposite manner, however, with the gas flowing through the inside of the tubes, preferably lined with the membrane, while the potassium formate solution contacts the tube on its shell or exterior side. The membrane may be coated or otherwise placed on either the inside or the outside of the permeable tube support. Generally, however, it is desirable for the gas to contact the membrane directly, so the flow of moisture through the membrane will be less likely to dislodge or erode the membrane from the tube surface at a weakly adhering point. Where the pressure difference between the gas and solution is of little consequence, a support need not be usedxe2x80x94that is, the membrane is self-supporting and the moisture is transmitted directly through it without having also to traverse a permeable support. In any case, the potassium or other alkali metal formate solution, having been diluted by the absorbed moisture, is then regenerated in any suitable manner.
Regeneration of the diluted solution of potassium or other alkali metal formate is simply the removal of water. Regeneration can be performed in a generally known manner by a reboiler or, preferably, a shock wave regenerator (sometimes known as a cavitation pump), as described in Sajewski""s U.S. Pat. Nos. 5,183,513, 5,184,576 and 5,239,948 and Griggs"" U.S. Pat. Nos. 5,385,298, 5,957,122 and 5,188,090, all of which are incorporated herein by reference in their entireties. A preferred cavitation regenerator is based on these patents and may be obtained from Hydro Dynamics, Inc. of Rome, Ga. Regeneration can also be performed by a membrane separator utilizing a membrane selected for its ability to transmit water from the dilute solution to the permeate side while retaining the potassium formate.
For the gas dehydration step I may use a membrane in any physical form which permits contacting the gas to be dehydrated on one side and the potassium or other alkali metal formate solution on the other side. The structure may be tubular, laminar, or of any other suitable type, or comprise the entire structure, as a hollow tube, and the membrane may be held by a separate permeable support or not; the solution may be inside a tube or outside (where there is a permeable support for the membrane, the gas is usually under pressure on the same side); in any case the contact of the gas on one side of the membrane and the potassium or other alkali metal formate solution on the other side of the membrane may be continuous, batch, countercurrent, or otherwise suitably arranged. Suitable membranes are described by Woodard in U.S. Pat. No. 5,632,805, Auvil et al in U.S. Pat. No. 5,259,869, Fournie et al in U.S. Pat. No. 4,497,640 and Makino et al in U.S. Pat. No. 4,718,921, and particularly Gore in U.S. Pat. No. 3,953,566, the concepts of which are useful in my invention in their entireties; in their background sections as well as their new disclosures, these patents describe membranes and devices which persons skilled in the art will recognize as having compositions and configurations generally useful in my invention.
Any membrane capable of passing moisture from natural gas to a liquid absorbent for water may be used in my invention. Preferably the membrane will have a moisture transmission rate exceeding 1000 g/m2/day, will permit no detectable flow of liquid water at hydrostatic pressures up to 172 kN/m2/day, and will exclude hydrocarbons such as methane from transmission. While liquid glycols could be used as the liquid absorbent because of their ability to absorb moisture, I prefer to use alkali metal formate solutions; potassium formate solutions are preferred primarily for environmental reasons.